Fourmolu

Fourmolu is a free, open-source formatter for Haskell source code, best suited to developers and teams who want configurable formatting built into their existing workflow. Its project-aware formatting and editor and CI options make it practical for routine use, but projects relying on complex CPP or CRLF line endings should look carefully at its limits.

Overview

As a configurable fork of Ormolu, Fourmolu uses GHC’s parser and aims to keep changes small, produce the same output on repeat runs, and handle large projects and modern Haskell dialects. Those goals make it a sensible choice for teams introducing formatting to an established codebase: smaller diffs are easier to review, while idempotence helps keep routine formatting from creating churn.

Settings can be stored in fourmolu.yaml or passed as command-line options. Fourmolu also locates the relevant Cabal file to read default language extensions and dependencies, helping it determine operator fixities in the context of the project. That Cabal awareness is useful when formatting depends on project-specific language settings, but it also makes the tool most compelling within a Cabal-based Haskell workflow.

Key features

• Configurable style: Controls cover indentation, line length, import grouping, record layout, Haddock comments, Unicode output, and operator fixities. Teams can shape the formatter around meaningful project conventions rather than accepting a single fixed style.
• Selective formatting: Magic comments can exempt selected code, and command-line options can target a specified line region. This gives developers a way to work around code that should not be reformatted or limit changes to part of a file.
• Browser demo: The interactive web app offers configurable formatting and program options, providing a way to explore results without first installing the formatter.
• Workflow integration: The README describes editor integration through Haskell Language Server by setting haskell.formattingProvider to fourmolu, and recommends the run-fourmolu GitHub Action for keeping projects formatted. These options help teams make formatting part of editing and CI rather than a separate manual step.
• Installation choices: The project recommends GHCup and also documents installation through Cabal, Stack, and dotslash, leaving Haskell developers several ways to add it to a project environment.

Pricing

Fourmolu is free: its open-source plan costs nothing. There is no paid tier or trial to weigh, making it suitable for individual developers and teams that need a Haskell formatter without a software fee. The trade-off is that users choose it for its formatting capabilities and workflow fit, not for a paid support or service tier.

Platforms

The browser demo makes Fourmolu usable on the web, while the project supports Linux and Windows. Its library API is also available for integration. The installation and editor options make it particularly relevant to Haskell development environments; macOS is not among the supported platforms stated for this entry.

Who it's for

Fourmolu is a strong fit for Haskell developers who want configurable project-wide rules, Cabal-aware handling of extensions and operator fixities, and editor or CI integration. Teams that value smaller formatting diffs and repeatable output should find its design goals relevant. It is less suitable for codebases that depend on complex CPP conditionals or require CRLF-preserving formatting.

Pros and cons

Pros

• Project-aware formatting: Reading Cabal defaults and dependencies helps the formatter account for language extensions and operator fixities.
• Useful style control: Options span common layout and presentation choices, with magic comments and region formatting for selective work.
• Multiple integration paths: Browser, editor, CI, and several installation methods can bring formatting into different stages of a Haskell workflow.
• No software cost: The open-source formatter is free for individual and team use.

Cons

• CPP support is experimental: It works only in simple cases where conditionals surround top-level declarations, limiting its fit for projects with more complex CPP use.
• LF-only output: Fourmolu always writes LF line endings, so check mode can fail on otherwise formatted files using CRLF.
• Haskell-specific: It formats Haskell source, so teams working in other languages need a different tool for those files.

Alternatives

Ormolu is the closest alternative for teams seeking a free, open-source Haskell formatter; choose between the two based on the configuration and workflow you need. For other language ecosystems, Black is a free Python code formatter, Fantomas formats F# source as a .NET tool, goimports is a free command-line tool, and rustfmt is an alternative for Rust projects. Prettier is another open-source code formatter with enterprise support and maintenance offered through Tidelift Subscription. Ruff and Accessibility Test Framework for Android are also listed as alternatives.

Browse more options in Code Formatting Tools.

Verdict

Choose Fourmolu if you maintain Haskell code and want configurable, Cabal-aware formatting that can fit into editor and CI workflows without a license fee. Its project-aware behavior and selective formatting options are persuasive strengths. Look elsewhere if your code depends on complex CPP or if CRLF output is a requirement.